A hydrogen explosion has rocked a nuclear power plant in Japan damaged by a massive earthquake and tsunami. At least six people were injured in the blast at the No. 3 reactor of the Fukushima Daiichi plant, where authorities have been working desperately

WATCH: A second explosion at the Fukushima Daiichi nuclear plant, around 240 kilometers from Tokyo, did heavy damage to the building around Reactor No. 3. Reactor No. 1 had already been damaged in a similar explosion, but authorities said containment vessels withstood the blast. (Reuters video)

Japanese authorities are racing to prevent a catastrophic meltdown at the Fukushima nuclear power plant, which was badly damaged in the March 11 earthquake and tsunami.

Officials from Tokyo Electric Power Company said the fuel rods appear to be melting inside all three of the most damaged nuclear reactors after they were exposed when a steam vent failed to open properly.

It is the second time fuel rods at the reactor have become exposed. Earlier on March 14, officials frantically pumped seawater into the second reactor in an attempt to cool the rods.

If the 4-meter rods remain exposed too long, the temperatures in the core could rise to a level where they melt through the protective steel walls.

The containment vessel that encases the core is supposed to protect the outside environment from any crisis inside, but the 9-magnitude earthquake that shook the country last week may have compromised the structural integrity of the vessel.

But in a statement, the Vienna-based head of the UN nuclear watchdog agency, Yukiya Amano, said the reactor vessels are intact and so far the radiation release has been "limited."

"The nuclear plants have been shaken, flooded, and cut off from electricity," Amano said. "Operators have suffered personal tragedies. But the reactor vessels have held and radioactive release is limited."

Amano said Japan has officially asked it to send a team of experts to help it deal with the crisis.

Radiation Released

The Japanese government has told citizens that the risk of a major radioactive leak remains small. Some 200,000 people have been evacuated from a 20-kilometer radius around the plant.

Low-level radiation has already been released by explosions on March 12 and early on March 14, and the wind over the quake-damaged nuclear complex was blowing south on March 14 toward the capital, Tokyo, 240 kilometers south.

But Japan's Meteorological Agency says the winds will be slow and that what has leaked so far is not expected to affect Tokyo.

On March 12, an explosion blew the roof off of the nuclear plant's Reactor No. 1 after the earthquake triggered an automatic shutdown of all six reactors.

A woman cries while sitting on a road amid the destroyed city of Natori, Miyagi Prefecture, on March 13.

Diesel-powered emergency backup generators were meant to keep the reactor cores cool during the emergency shutdown. But those generators were flooded and knocked out when the tsunami struck the facility on the northeastern coast of Japan, leaving only emergency batteries to run the cooling system.

Cabinet Secretary Yukio Edano says it appears that an earlier explosion on March 14 in Reactor No. 3 was triggered when the backup battery power became depleted there and authorities pumped in seawater in a desperate attempt to keep the core from overheating.

"The explosion was believed to be the same sort of explosion as at the Reactor No. 1," Edano said. "We had issued an evacuation order for people living within a 20-kilometer radius of the plant and [now] we have told people who were in the process of evacuating to go indoors immediately."

'Hydrogen Explosion'

Kaoru Yoshida, a spokesman for Tokyo Electric Power Company, described the blast at Reactor No. 3 as a "hydrogen explosion."

"There was a large sound from Reactor No. 3 at 11:01 a.m. and white smoke rose," Yoshida said. "We think it is a hydrogen explosion."

But officials from the International Atomic Energy Agency said they do not think the concrete containment vessels around the two reactors were breached -- a worst-case scenario that would lead to a major radioactive leak similar to the 1986 Chornobyl nuclear power plant disaster in what was then Soviet Ukraine.

In Brussels, the European Commission called an emergency meeting of all 27 energy ministers in the European Union for March 15 to discuss nuclear safety in the wake of Japan's power plant disaster.

A skull placard left by antinuclear protesters near the Reichstag and the Chancellory in Berlin on March 14.

EU Energy Commissioner Gunther Oettinger also has invited national nuclear safety authorities and nuclear power plant operators from across the EU to discuss safety requirements for earthquakes and emergency power supply systems for reactor cooling.

Several European countries have announced measures to revise their atomic energy policies in the wake of the crisis.

German Chancellor Angela Merkel said a decision made last year to extend the life of the country's 17 nuclear power stations would be suspended for three months. A previous government decided a decade ago to shut all 17 German nuclear plants by 2021, but Merkel's administration in 2010 moved to extend their lives by an average 12 years.

Switzerland suspended its plans to build and replace nuclear plants, while Austria's environment minister called for atomic stress tests to make sure Europe's nuclear facilities are "earthquake-proof."

Lifespan Debate

The disaster in Japan also is expected to trigger debate on the lifespan of nuclear reactors. The quake-damaged Fukushima power plant was built 40 years ago and had only one more month of operation in its current lifespan before requiring a new permit to continue operations for another 20 years. Authorities who issue those permits say they halted the permit renewal process for Fukushima on March 14.

Meanwhile, millions of people spent a third night without water, food, or heating in near-freezing temperatures along the devastated northeastern coast. There have been more than 150 aftershocks since the March 11 quake.

Rescue workers were using chainsaws and hand picks today to dig out bodies from coastal towns devastated by the earthquake and tsunami as the official death toll from the disaster climbs.

Some 2,000 bodies were discovered along the northeastern coastline on March 14 -- victims of the tsunami. That raises the official death toll from the disaster to more than 2,800. But tens of thousands of people are still missing -- including 18,000 people from one town there.

Japan also is suffering economically as a result of the disasters with share prices on the Tokyo Stock Exchange plunging today due to investors' fears of huge losses by Japanese industries -- including global companies like Toyota and Honda.

written by Ron Synovitz in Prague, with contributions from Rikard Jozwiak in Brussels and agency reports